meson: fix TLS support under mingw
GCC on Windows complains that "__declspec(thread)" doesn't work, but still compiles it, so the meson check doesn't work. The warning printed by gcc: "warning: 'thread' attribute directive ignored [-Wattributes]"
Pass -Werror=attributes to make the check fail instead.
This fixes the test suite (minus gtk tests) on Windows with mingw.
Edited by Christoph Reiter